Recently, an X-ray Computed Tomography (CT) apparatus uses a scanning method by continuously rotating an X-ray tube in a circular orbit about a subject and continuously reciprocating a top plate (for example, see JP-A H6-1258889 (KOKAI)). According to such scanning method, an X-ray is helically radiated onto the subject, so that tomographic images in a wide area with a good continuity can be obtained. Hereinafter, such scanning method is referred to as a “helical shuttle scan”.
According to the helical shuttle scan, orbits of X-rays to be helically radiated onto a subject are controlled so as to be synchronized between a plurality of projection data obtained by scanning in a going way (hereinafter, “going-way scan”), or between a plurality of projection data obtained by scanning in a return way (hereinafter, “return-way scan”). Such control is hereinafter referred to as “orbit synchronization control”. For example, conventionally, an X-ray CT apparatus performs orbit synchronization control so as to start radiation of an X-ray based on an origin in a circular orbit (rotational angle 0°) as a starting point in both a going-way scan and a return-way scan. In other words, the X-ray CT apparatus starts radiation in a going-way scan when an X-ray tube is at the rotational angle 0°, and starts radiation in a return-way scan also when the X-ray tube is at the rotational angle 0°.
However, when returning from the going-way scan to the return-way scan, radiation needs to be started after waiting the X-ray tube moves to the position of the origin in the circular orbit (the rotational angle 0°), resulting in a problem that a time loss is large. For example, when radiation in a going-way scan is completed, and a preparation for starting a return-way scan is completed, if the X-ray tube is just past the origin (the rotational angle 0°), the X-ray CT apparatus needs to wait substantially one round until the X-ray tube moves to the origin again, and then to start radiation. It is similar in another going-way scan after the return-way scan is completed.